NGU - BULL 427. 1995 Tom Heldal & Bjem Lund 123 A regional study of the dimension-stone potential in labradorite-bearing anorthositic rocks in the Rogaland Igneous Complex TOM HELDAL & BJORN LUND Tom Heldal & Biem Lund, Norges geologiske undersoketse, P.D. Box 3006 Lade, N-7002 Trondheim, Norway Introduction 1961, Duchesne & Maquil 1987, Duchesne & The occurrence of anorthositic rocks containing Michot 1987). The Bjerkreim-Sokndal layered irridescent feldspar in the Rogaland Igneous lopolith comprises a suite of anorthositic-mange­ Complex has been known for almost a century. ritic rocks (Duchesne 1987), while the Garsaknat Until recently, however, no study of the econo­ and Hidra intrusions are massive anorthosites mic potential of these rocks as dimension-stone with leuconoritic margins (Demaiffe & Hertogen resources has been carried out, even though 1981, Duchesne & Michot 1987). such rocks are highly valued on the international market due to their attractive play of colours. General aspects The scope of the present regional study was From an industrial point of view, the main poten­ to identify and characterise rock-types and units tial for dimension-stone in the Rogaland Igneous containing such feldspar and make an inventory Complex is where massive bodies of anorthosite of the dimension-stone potential in the igneous and leuconorite containing labradorescent plagi­ complex (Heldal & Lund 1994). The project was oclase are found. This view is strictly related to initiated by NGU, Rogaland Nreringstjeneste N S subjective preferences in the dimension-stone and the local authorities of Dalane, and carried market, where rock-types showing unique colour out by NGU in 1993/94 with financial support or a play of colours have higher value than less from local and regional governments. unique rock types. The anorthositic rocks of the Rogaland In anorthosites the play of colour (Iabradore­ Igneous Complex comprise four anorthositic scence) is produced by lamellar structure in massifs (Egersund-Ogna, Haland, Helleren and labradorite, known as Boggild intergrowths Ana-Sira), a layered lopolith (Bjerkreim-Sokndal) (Boggild 1924). These consist of lamellae (0.1 ­ and two smaller intrusive bodies (Garsaknat and 0.25 microns) of two plagioclases which may dif­ Hidra; Fig. 1). The massif-type anorthosites are fer in composition by perhaps 12% An ( Smith essentially composed of monotonous anorthosite 1974). The compositional range of plagioclase in and leuconorite in various proportions (Michot which labradorescence occurs is believed to be between An47 and An58' and the colour of irride­ scence may vary from blue (An47-55) to green/yellow (An50-55) and red (An55.58) (Smith 1974, Zeino-Mahmalat 1972). The plagioclases of the anorthosite massifs in Rogaland generally range in composition from An40 to An56 (Zeino-Mahmalat 1972, Duchesne & Maquil 1987), but the labradorescent type (An>47) only occurs within limited parts. Although occurrences are found in all the descri­ bed massifs and intrusions, the potential for wor­ kable deposits seems to be limited to the Egersund-Ogna Massif and the Hidra and Garsaknat intrusions. This limitation is based mainly on other industrial and geological factors of importance, e.g. accessibility, homogeneity and fracturing. Generally, the existence of labra­ Fig. 1. Anorthosltlc rocks in the Rogaland Igneous Complex (simplified after Duchesne & Michot 1987). 1 Egersund-Ogna dorite is closely associated with pyroxene, as massif; 2 HAland massif; 3 Helleren massif; 4 Bjerkreim­ megacrysts or evenly distributed grains, and rock Sokndal lopolith/monzonoritic intrusions; 5 Ana-Sira massif; 6 composition varies from anorthosite to leuconori­ Hidra intrusion; 7 Garsaknat intrusion. te. 124 Tom Heldal & Bjom Lund GU . BUll 427. 1995 surrounding melt during the final emplacement of the massif (Robins pers. comm. 1994). Grain size varies from medium to coarse to very coarse, and the plagioclase generally shows a bimodal distribution with large, subhe­ dral crystals up to 5cm in size in a finer grained matrix. Between 10% and 30% of the plagioclase crystals show blue to green labradorescence. P.yr~xene « 2 ~%) commonly occurs as evenly distributed grains or as megacrysts, in places concentrated in boudinaged layers. The content of opaq~es . (scattered, small grains of pyrite and magnetite) IS generally less than 0.5%, indicating that the rock would be well suited for most buil­ ding stone purposes. This view is supported by the nature of the weathering profile, which is very shallow and almost without staining. Saussuritisation and kaolinisation are com­ mon phenomena in anorthosites, and the degree and nature of such alteration are of great impor­ tance for exploitation as for use of dimension sto­ ne. Generally, highly altered rocks are not corn­ ~ e r ci ally interesting. In the Hellvik area it is pos­ sible to distinguish between regional and local alteration. The former is mainly seen in the monotonous anorthosites, as small white and green spots on a mesoscopic scale, and as 'dus­ ty' plagioclase microscopically. The labradorite­ bearing anorthosites, however, are remarkably unaltered. Local alteration occurs along fracture zones ( co m ~ o nl y WNW-ESE and N-S), and a gradual transition from unaltered rock to white rneta­ anorthosite is seen when approaching these. Fig. 2 (Upper photo). Polished sample of Hellvik-type anortho­ site (prospect 5). Scale 1:1. (l ower photo). Polished sample of The frequency of fracture zones also controls the Hidra-type anorthosite (prospect 9). Scale 1:1. j ~i n ti n g of the rocks in between, and wide spa­ cing ~f fracture zones implies large deposits of massive anorthosite/leuconorite. Thus, the exploitable parts of the bodies occur as individu­ On the basis of colour, it is convenient to dis­ al, irregular blocks of 'fresh' rock between the tinguish two types of labradorite-bearing anor­ fracture zones and bordered by homogeneous thosite!leuconor ite: the Hellvik type and the Hidra anorthosite. In the lowlands close to the sea, the type . The former is brownish red with a clear frequency of fracture zones is generally higher blue to green play of colour, while the latter is than in the hills in the northern part of the area dark grey with a deep blue labradorescense and r e s u ! tin~ in a better potential for large block p ro ~ is rich in iron oxides (Fig. 2). An important factor duction In the north. At several places, spacing of r e~ ar~ i n g the variation in colour is probably the fracture zones is more than 50 m, and of visible oXld.atlon state of the rocks (Smith 1974), which joints 5 m or more. again may be related to cooling mechanisms and In all the deposits, however, the occurrence of metamorph ism. pyroxene megacrysts and clusters of thin gree­ The Hellvik type occurs in the central part of the nish veins influences around 20-50% of the rock Egersund-Ogna Massif close to the coast near volume, contributing to a higher quarrying waste Hellvik, as irregular bodies surrounded by homo­ rat io th an th e joint and fracture pattern a lone g en eous , andesine-bearing anorthosite (Fi g . 3). would indicate. Contacts are intrusive, sharp or transitional and Five prospects are located in the Hellvik area the irregular bodies are believed by some' aut­ (1-5 o.n F i~ . 3). P r ~s p ec t s 1 and 2 are large ~ors to r ep r~sen t fragments from an early stage deposits With potential for large-scale extraction In the evolution of the complex, enclosed in the NGU - BULL 427. 1995 Tom Heldal & Bjorn Lund 125 tals on the island of Hidra, are well known among collectors, but probably not suitable as dimensi­ on stone resources. For that purpose, large and homogeneous deposits of finer grained material would be of greater interest. The Hidra type is characterised by an overall dark grey colour with scattered labradorite crys­ tals. The bimodal distribution of the feldspar is even more significant than in the Hellvik type , with labradorite megacrysts surrounded by a medium- to coarse-grained matrix. The conten t of labradorescent plagioclase varies between 1% and 10% in the homogeneous anorthosites and slightly more in the pegmatites. The play of colours is mainly towards deep blue. These anorthosites and leuconorites have a high content of opaques (mainly haematite but also magnetite and pyrite), locally up to 10%. This results in a dee p, stained, weathering profile and clearly limits suitability for outdoo r use of the stone . Furthermore, the weathered surface makes correct quantitative evaluation of labrado­ rescence difficult, and for detailed investigation core-drilling or blasting will be necessary to obtain fresh samples. Four prospects are localised - 6-7 in the Garsaknat intrusion and 8-9 in the Hidra intrusi ­ on (Fig. 4). Prospects 6 and 9 are relatively hete­ rogeneous, with irregular pegmatite bodies com­ monly occurring in finer grained anorthosite and leuconorite. The appearance in several places is very good , with a high content of deep blue labradorescent crystal s. However, due to litholo­ gical variations and fracturing, large blocks of Railway uniform quality are not likely to be obtained. r++l labradorile-bearing ~ anorthostte and teucononte Road R44 Larger deposits of homogeneous , massive anorthosite are found at prospects 7 and 8. Norilic dyke Track Labradorescent crystals occur throughout these Fracture zones C:> Prospect 1-5 areas , but the quant ity seems to be lower than at prospects 6 and 9; and possibly too low for gai­ Fig. 3. Distribution of exploitable deposits of anorthositic rocks ning a good market value . Furthermore, thin gre­ with labradorescent plagioclase in the Hellvik area. central part of the Egersund-Ogna Massif. See Fig. 1 for localisation of enish veins occur in many places . However, the map. Prospects are numbered. potential for large-scale extraction of large blocks in these deposits provides good reason for fur­ of large blocks of overall good quality.